Dielectric waveguide and method of making the same

ABSTRACT

In general, in one aspect, the invention features a waveguide that includes a first portion extending along a waveguide axis including a first chalcogenide glass, and a second portion extending along the waveguide axis including a second chalcogenide glass, wherein the second chalcogenide glass is different from the first chalcogenide glass.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to Provisional PatentApplication 60/428,382, entitled “HIGH POWER WAVEGUIDE,” and filed Nov.22, 2002, and Provisional Patent Application 60/458,645, entitled“PHOTONIC CRYSTAL FIBER,” and filed Mar. 28, 2003, the entire contentseach of which are hereby incorporated by reference.

BACKGROUND

[0002] This invention relates to the field of dielectric waveguides andmethods for making waveguides.

[0003] Waveguides play important roles in numerous industries. Forexample, optical waveguides are widely used in telecommunicationsnetworks, where fiber waveguides such as optical fibers are used tocarry information between different locations as optical signals. Suchwaveguides substantially confine the optical signals to propagationalong a preferred path or paths. Other applications of opticalwaveguides include imaging applications, such as in an endoscope, and inoptical detection.

[0004] The most prevalent type of fiber waveguide is an optical fiber,which utilizes index guiding to confine an optical signal to a preferredpath. Such fibers include a core region extending along a waveguide axisand a cladding region surrounding the core about the waveguide axis andhaving a refractive index less than that of the core region. Because ofthe index-contrast, optical rays propagating substantially along thewaveguide axis in the higher-index core can undergo total internalreflection (TIR) from the core-cladding interface. As a result, theoptical fiber guides one or more modes of electromagnetic (EM) radiationto propagate in the core along the waveguide axis. The number of suchguided modes increases with core diameter. Notably, the index-guidingmechanism precludes the presence of any cladding modes lying below thelowest-frequency guided mode for a given wavevector parallel to thewaveguide axis. Almost all index-guided optical fibers in usecommercially are silica-based in which one or both of the core andcladding are doped with impurities to produce the index contrast andgenerate the core-cladding interface. For example, commonly used silicaoptical fibers have indices of about 1.45 and index contrasts rangingfrom about 0.2% to 3% for wavelengths in the range of 1.5 μm, dependingon the application.

[0005] Drawing a fiber from a preform is the most commonly used methodfor making fiber waveguides. A preform is a short rod (e.g., 10 to 20inches long) having the precise form and composition of the desiredfiber. The diameter of the preform, however, is much larger than thefiber diameter (e.g., 100's to 1000's of times larger). Typically, whendrawing an optical fiber, the material composition of a preform includesa single glass having varying levels of one or more dopants provided inthe preform core to increase the core's refractive index relative to thecladding refractive index. This ensures that the material forming thecore and cladding are rheologically and chemically similar to be drawn,while still providing sufficient index contrast to support guided modesin the core. To form the fiber from the preform a furnace heats thepreform to a temperature at which the glass viscosity is sufficientlylow (e.g., less than 10⁸ Poise) to draw fiber from the preform. Upondrawing, the preform necks down to a fiber that has the samecross-sectional composition and structure as the preform. The diameterof the fiber is determined by the specific rheological properties of thefiber and the rate at which it is drawn.

[0006] Preforms can be made using many techniques known to those skilledin the art, including modified chemical vapor deposition (MCVD), outsidevapor deposition (OVD), plasma activated chemical vapor deposition(PCVD) and vapor axial deposition (VAD). Each process typically involvesdepositing layers of vaporized raw materials onto a wall of a pre-madetube or rod in the form of soot. Each soot layer is fused shortly afterdeposition. This results in a preform tube that is subsequentlycollapsed into a solid rod, over jacketed, and then drawn into fiber.

[0007] Optical fibers applications can be limited by wavelength andsignal power. Preferably, fibers should be formed from materials thathave low absorption of energy at guided wavelengths and should haveminimal defects. Where absorption is high, it can reduce signal strengthto levels indistinguishable from noise for transmission over longfibers. Even for relatively low absorption materials, absorption by thecore and/or cladding heats the fiber. Defects can scatter guidedradiation out of the core, which can also lead to heating of the fiber.Above a certain power density, this heating can irreparably damage thefiber. Accordingly, many applications that utilize high power radiationsources use apparatus other than optical fibers to guide the radiationfrom the source to its destination.

SUMMARY

[0008] High power laser systems are disclosed. Such systems operate atpowers of at least about one Watt. In some cases, operational intensitycan be more than about 100 Watts, such as about a kilowatt or more.These systems include dielectric waveguides for delivering the laserbeam to a target. The energy guided by the waveguides can have extremelyhigh power densities. For example, the power density in some waveguidescan be more than about 10⁶ W/cm² (e.g., more than about 10⁸ W/cm², morethan about 10¹⁰ W/cm²).

[0009] Suitable dielectric waveguides include fiber waveguides capableof guiding high power electromagnetic energy, such as certain photoniccrystal fibers (e.g., certain Bragg fibers). Such dielectric waveguidesinclude one or more portions formed from a chalcogenide glass. In someembodiments, the dielectric waveguides can include two (or more)different chalcogenide glasses, where the different chalcogenide glasseshave different refractive indexes. Note that the refractive index of amaterial refers to the refractive index of a material at the wavelengthat which the waveguide is designed to guide light. Preferably, thedifferent glasses have similar thermomechanical properties and can beco-drawn.

[0010] The portions of the waveguide are structural elements of thewaveguide that determine the optical properties of the waveguide (e.g.,structural elements that determine how the waveguide confines an opticalsignal to a path). In preferred embodiments, the fiber waveguide is aphotonic crystal fiber, which includes a core and a confinement region.The confinement region has a refractive index variation that forms abandgap and reflects light within a certain range of frequencies,confining that light to the core. One type of photonic crystal fiber isa Bragg fiber, in which the confinement region can include multiplelayers of different composition that give rise to the index variation.In such cases, each of the layers is considered a portion of thewaveguide.

[0011] Photonic crystal waveguides can have hollow cores, which isadvantageous in high power applications because absorption of guidedenergy by the core (and subsequent heating) is significantly reducedcompared to a solid core waveguide.

[0012] In some embodiments, the dielectric waveguides are configured toguide electromagnetic energy at infrared wavelengths (e.g., betweenabout 1 micron and 15 microns, between about 5 microns and 12 microns,such as about 10.6 microns). The materials forming the waveguides (e.g.,chalcogenide glasses) may have relatively low absorption at thesewavelengths compared to other materials, such as some other glasses.Thus, use of chalcogenide glasses at these wavelengths can beadvantageous because they may have lower loss than similar waveguidesformed from other materials (e.g., polymers or oxide glasses), makingthem suitable for guiding output energy from the high power laser to thetarget.

[0013] Methods for making dielectric waveguides are also disclosed. Inparticular, chemical vapor deposition (CVD) methods suitable fordepositing layers of different materials in a deposition tube aredisclosed. These methods can be used, for example, to depositalternating layers of two different chalcogenide glasses in a depositiontube or to deposit alternating layers of a chalcogenide glass and anoxide glass. CVD methods can provide preforms that can be drawn intofibers with low defect densities. Because defects tend to scatterenergy, which locally heats the fiber, low defect density fiber isparticularly desirable for high power density transmission whereexcessive heating can be fatal to the fiber.

[0014] In general, in a first aspect, the invention features a waveguidethat includes a first portion extending along a waveguide axis includinga first chalcogenide glass, and a second portion extending along thewaveguide axis including a second chalcogenide glass, wherein the secondchalcogenide glass is different from the first chalcogenide glass.

[0015] Embodiments of the waveguide can include one or more of thefollowing features and/or features of other aspects.

[0016] The first chalcogenide glass can have a different refractiveindex than the second chalcogenide glass. The first chalcogenide glasscan include As and Se. For example, the first chalcogenide glass caninclude As₂Se₃. In some embodiments, the first chalcogenide glass canfurther include Pb, Sb, Bi, I, or Te. The second chalcogenide glass caninclude As and S (e.g., As₂S₃), and/or P and S. The second chalcogenideglass can include Ge or As.

[0017] The first chalcogenide glass can have a refractive index of 2.7or more. The second chalcogenide glass has a refractive index of 2.7 orless. The first chalcogenide glass can have a glass transitiontemperature (T_(g)) of about 180° C. or more. The second chalcogenideglass can have a T_(g) of about 180° C. or more.

[0018] The waveguide can have a loss coefficient less than about 2 dB/mfor electromagnetic energy having a wavelength of about 10.6 microns.The waveguide can have a hollow core. The first portion can surround acore (e.g., the hollow core). The second portion can also surround thecore. The second portion can surround the first portion. The core canhave a minimum cross-sectional dimension of at least about 10 λ (e.g.,about 20 λ, 50 λ, 100 λ), where λ is the wavelength of radiation guidedby the waveguide. The core can have a minimum cross-sectional dimensionof at least about 50 microns (e.g., at least about 100 microns, at leastabout 200 microns).

[0019] The waveguide can be a photonic crystal fiber, such as a Braggfiber. The photonic crystal fiber can include a confinement region andthe first and second portions are part of the confinement region.

[0020] In general, in another aspect, the invention features a methodthat includes providing a waveguide having a first portion extendingalong a waveguide axis including a first chalcogenide glass and a secondportion extending along the waveguide axis, and guiding electromagneticenergy from a first location to a second location through the waveguide.

[0021] Embodiments of the method can include one or more of thefollowing features, and/or features of other aspects.

[0022] The second portion can include a second chalcogenide glassdifferent from the first chalcogenide glass. The electromagnetic energycan have a wavelength of between about 2 microns and 15 microns. Theelectromagnetic energy can have an intensity of more than about one Watt(e.g., more than about 5 Watts, 10 Watts, 50 Watts, 100 Watts, such as 1kW or more).

[0023] The method can include coupling the electromagnetic energy from alaser into the waveguide. The laser can be a CO₂ laser.

[0024] The waveguide can be a photonic crystal fiber, such as a Braggfiber.

[0025] In general, in a further aspect, the invention features anapparatus that includes a dielectric waveguide extending along an axisand configured to guide electromagnetic radiation along the axis,wherein the electromagnetic radiation has a power greater than about 1Watt.

[0026] Embodiments of the apparatus can include one or more of thefollowing features and/or features of other aspects.

[0027] The electromagnetic radiation can have a wavelength greater thanabout 2 microns (e.g., greater than about 5 microns). Theelectromagnetic radiation can have a wavelength less than about 20microns (e.g., less than about 15 microns). For example, theelectromagnetic radiation can have a wavelength between about 10 micronsto 11 microns (e.g., about 10.6 microns).

[0028] The electromagnetic radiation can have a power greater than about5 Watts (e.g., greater than about 10 Watts, 50 Watts, 100 Watts, such as1 kW or more).

[0029] The dielectric waveguide can include a first portion extendingalong the waveguide axis including a first chalcogenide glass. Thedielectric waveguide can further include a second portion extendingalong the waveguide axis, the second portion having a differentcomposition than the first portion. The second portion can include anoxide glass or a chalcogenide glass. For example, the second portion caninclude a second glass different from the first chalcogenide glass.

[0030] The waveguide can be a photonic crystal fiber, such as a Braggfiber. The waveguide can have a hollow core.

[0031] In general, in another aspect, the invention features a methodthat includes exposing a surface to a first gas composition underconditions sufficient to deposit a layer of a first chalcogenide glasson the surface, and exposing the layer of the first chalcogenide glassto a second gas composition under conditions sufficient to deposit alayer of a second glass on the layer of the first chalcogenide glass,wherein the second glass is different from the first chalcogenide glass.

[0032] Embodiments of the method can include one or more of thefollowing features and/or features of other aspects.

[0033] Exposing the surface to the first gas composition can includeactivating a plasma in the first gas composition. Activating the plasmacan include exposing the gas to electromagnetic radiation to activatethe plasma (e.g., microwave or radio frequency radiation).

[0034] Exposing the layer of the first chalcogenide glass to the secondgas composition can include activating a plasma in the second gascomposition, which can include exposing the second gas composition toelectromagnetic radiation to activate the plasma (e.g., microwave orradio frequency radiation).

[0035] The second gas composition is typically different from the firstgas composition. The first gas composition can include one or morehalide compounds (e.g., one or more chloride compounds). The first gascomposition can include a carrier gas (e.g., nitrogen or a noble gas,like argon). The first gas composition can include a chalcogen. Thefirst gas composition pressure can be between about 2 and 20 Torr.

[0036] The second gas composition can include one or more halidecompounds (e.g., chloride compounds). The second gas composition caninclude a carrier gas (e.g., nitrogen or a noble gas, like argon). Thesecond gas composition can include a chalcogen. Alternatively, oradditionally, the second gas composition comprises oxygen. The secondgas composition pressure can be between about 2 and 20 Torr.

[0037] The second glass can be an oxide glass or a chalcogenide glass.

[0038] The surface can be a surface of a tube, e.g., an inner surface ofa tube. The tube can be a glass (e.g., an oxide glass, such as asilicate glass) tube or a polymer tube. In some embodiments, the surfaceis a planar surface.

[0039] In general, in a further aspect, the invention features a methodthat includes introducing a first gas composition into a tube, the firstgas composition including a first compound that is substantially inertwith respect to a first material forming the inner surface of the tube,and exposing the first gas composition to conditions sufficient tochange the first compound into a second compound reactive with the firstmaterial and to deposit a layer of a second material on the innersurface of the tube.

[0040] Embodiments of the method can include one or more of thefollowing features and/or features of other aspects.

[0041] Exposing the first gas composition to conditions sufficient tochange the first compound into a second compound can include activatinga plasma in the first gas composition. In some embodiments, activatingthe plasma includes exposing the first gas composition toelectromagnetic radiation (e.g., microwave or radio frequencyradiation).

[0042] The first compound can include oxygen. For example, the firstcompound can be nitrous oxide. The second compound can be oxygen. Thefirst material can be a glass, such as a chalcogenide glass.

[0043] In some embodiments, the method further includes exposing thelayer of the first material to a second gas composition under conditionssufficient to deposit a layer of a second material on the layer of thefirst material, wherein the second glass is different from the firstglass. For example, the first glass can be a chalcogenide glass and thesecond glass can be an oxide glass.

[0044] Embodiments of the invention may include one or more of thefollowing advantages.

[0045] Waveguides disclosed herein can guide high intensityelectromagnetic radiation without sustaining damage due to heating.These waveguides can exhibit low loss at guided wavelengths.

[0046] The CVD techniques disclosed herein may be used to deposit layersof dissimilar materials (e.g., optically dissimilar) on a substrate. Insome embodiments, dissimilar materials can be deposited withoutsignificant undesirable reactions occurring between the gases used fordepositing a second material and the surface of the initially depositedmaterial. In some embodiments, the CVD process can deposit layers ofoptically dissimilar materials that have similar thermomechanicalproperties, and can be co-drawn. Waveguides formed using the CVD processcan have low defect densities (e.g., low impurity concentrations), andmay thus be particularly suitable for high power applications, wherehigh defect densities could result in significant heating (andultimately, failure) of the waveguide.

[0047] Unless otherwise defined, all technical and scientific terms usedherein have the same meaning as commonly understood by one of ordinaryskill in the art to which this invention belongs. In case of conflict,the present specification, including definitions, will control. Inaddition, the materials, methods, and examples disclosed herein areillustrative only and not intended to be limiting.

[0048] Additional features, objects, and advantages of the inventionwill be apparent from the following detailed description and drawings,and from the claims.

DESCRIPTION OF DRAWINGS

[0049]FIG. 1 is a schematic diagram of a laser system incorporating aphotonic crystal fiber.

[0050]FIG. 2 is a cross-sectional view of an embodiment of a photoniccrystal fiber.

[0051]FIG. 3A is a plot showing modeled radiation loss of a photoniccrystal fiber as a function of wavelength.

[0052]FIG. 3B is a plot showing modeled absorption loss of the photoniccrystal fiber as a function of wavelength.

[0053]FIG. 4 is a schematic diagram of a chemical vapor deposition (CVD)system.

[0054]FIG. 5 is a schematic diagram of a portion of the CVD system shownin FIG. 4.

[0055]FIG. 6 is a schematic diagram of a laser system incorporating aphotonic crystal fiber.

[0056] Like reference symbols in the various drawings indicate likeelements.

DETAILED DESCRIPTION

[0057] Referring to FIG. 1, a laser system 100 includes a laser 110 anda photonic crystal fiber 120 for guiding electromagnetic (EM) energyfrom the laser to a location 130 remote from the laser. Radiation iscoupled from laser 110 into fiber 120 using a coupler 140. Laser 110 canbe continuous wave or pulsed. The distance between laser 110 andlocation 130 can vary depending on the specific application, and can beon the order of several meters or more (e.g., more than about 10 m, 20m, 50 m, 100 m).

[0058] Laser system 100 can operate at UV, visible, or infrared (IR)wavelengths. In some embodiments, photonic crystal fiber 120 isconfigured to guide IR energy emitted by laser 110, and the energy has awavelength between about 0.7 microns and 20 microns (e.g., between about2 to 5 microns or between about 8 to 12 microns). In some embodiments,laser 110 is a CO₂ laser and the radiation has a wavelength of about 6.5microns or 10.6 microns. Other examples of lasers which can emit IRenergy include Nd:YAG lasers (e.g., at 1.064 microns) Er:YAG lasers(e.g., at 2.94 microns), Er, Cr: YSGG (Erbium, Chromium doped YttriumScandium Gallium Garnet) lasers (e.g., at 2.796 microns), Ho:YAG lasers(e.g., at 2.1 microns), free electron lasers (e.g., in the 6 to 7 micronrange), and quantum cascade lasers (e.g., in the 3 to 5 micron range.

[0059] The power emitted from laser 110 at the guided wavelength canvary. Although the laser power can be relatively low, e.g., mW, in manyapplications the laser system is operated at high powers. For example,the laser output intensity can be more than about one Watt (e.g., morethan five Watts, 10 Watts, 20 Watts). In some applications, the laseroutput energy can be more than about 100 Watts, such as several hundredWatts (e.g., about 200 Watts, 300 Watts, 500 Watts, 1 kilowatt).

[0060] For high power systems, the power density guided by fiber 120 canbe extremely high. For example, power density in the fiber can be morethan about 10⁶ W/cm², such as more than about 10⁷ W/cm², 10⁸ W/cm², 10⁹W/cm², or 10¹⁰ W/cm².

[0061] Fiber 120 can have relatively low losses at the guided wavelength(e.g., less than about 10 dB/m, 5 dB/m, 2 dB/m, 1 dB/m, 0.5 dB/m, 0.2dB/m). Due to the low loss, only a relatively small amount of the guidedenergy is absorbed by the fiber, allowing the fiber to guide high powerradiation without substantial damage due to heating.

[0062] Coupler 140 can be any coupler suitable for the wavelength andintensity at which the laser system operates. One type of a coupler isdescribed by R. Nubling and J. Harrington in “Hollow-waveguide deliverysystems for high-power, industrial CO₂ lasers,” Applied Optics, 34, No.3, pp. 372-380 (1996). Other examples of couplers include one or morefocusing elements, such as one or more lenses. Coupling efficiency canbe high. For example, coupler 140 can couple more than about 70% of thelaser output into a guided mode in the fiber (e.g., more than about 80%,90%, 95%, 98%). Coupling efficiency refers to the ratio of power guidedaway by the desired mode to the total power incident on the fiber.

[0063] Optionally, system 100 includes a cooling apparatus 150 (e.g., apump or compressor), which reduces heating of fiber 120 duringoperation. Cooling apparatus 150 can be an air-based system, forcing airthrough a sheath 165, which surrounds other portions of the fiber.Alternatively, cooling apparatus 150 can utilize a liquid coolant (e.g.,water), forcing a liquid through the sheath.. Cooling apparatus 150 maybe particularly beneficial in applications where the fiber guides energyat extremely high intensities (e.g., several hundred Watts orkilowatts). For example, the fiber may be maintained at temperatureswithin its operational range at such high intensities.

[0064] Referring to FIG. 2, photonic crystal fiber 120 includes a core220 extending along a waveguide axis and a dielectric confinement region210 (e.g., a multilayer cladding) surrounding the core. Confinementregion 210 is surrounded by a support layer 250, which providesmechanical support for the confinement region. Optionally, support layer250 is surrounded by sheath 165. A space 265 exists between sheath 165and fiber 120. As discussed previously, a liquid or gas can be forcedthrough the space between the sheath and the cladding to cool the fiberduring operation.

[0065] In the embodiment of FIG. 2, confinement region 210 is shown toinclude alternating layers 230 and 240 of dielectric materials havingdifferent refractive indices. One set of layers, e.g., layers 240,define a high-index set of layers having an index n_(H) and a thicknessd_(H), and the other set of layers, e.g., layers 230, define a low-indexset of layers having an index n_(L) and a thickness d_(L), wheren_(H)>n_(L) (e.g., n_(H)−n_(L) can be greater than or equal to orgreater than 0.01, 0.05, 0.1, 0.2, 0.5 or more). For convenience, only afew of the dielectric confinement layers are shown in FIG. 1. Inpractice, confinement region 210 may include many more layers (e.g.,more than about 15 layers, 20 layers, 30 layers, 40 layers, 50 layers,such as 80 or more layers).

[0066] Although not illustrated in FIG. 2, fiber 120 may include one ormore additional layers between the confinement region and the core. Forexample, the fiber may include one or more layers selected to tailor thedispersion characteristics of the fiber. Examples of such fibers aredescribed in U.S. patent application Ser. No. 10/057,440, entitled“PHOTONIC CRYSTAL OPTICAL WAVEGUIDES HAVING TAILORED DISPERSIONPROFILES,” filed Jan. 25, 2002, and having Pub. No. US-2002-0176676-A1,the entire contents of which are hereby incorporated by reference.

[0067] Layers 240 include a material having a high refractive index,such as a chalcogenide glass. The high index material in layers 240 canbe rheologically compatible with the material forming layers 230. Thematerial in each of layers 240 can be the same or different. Layers 230include a material having a refractive index lower than the high indexmaterial of adjacent layers 240, and can include a second chalcogenideglass or an oxide glass. In embodiments where layers 230 and 240 bothinclude chalcogenide glasses, the glasses are usually different. Thematerial in each of layers 230 can be the same or different. Examples ofhigh and low index materials are described below.

[0068] In the present embodiment, core 220 is hollow. Optionally, thehollow core can be filled with a fluid, such as a gas (e.g., air,nitrogen, and/or a noble gas) or liquid (e.g., an isotropic liquid or aliquid crystal). Alternatively, core 220 can include any material orcombination of materials that are Theologically compatible with thematerials forming confinement region 210. In certain embodiments, core220 can include one or more dopant materials, such as those described inU.S. patent application Ser. No. 10/121,452, entitled “HIGHINDEX-CONTRAST FIBER WAVEGUIDES AND APPLICATIONS,” filed Apr. 12, 2002and now published under Pub. No. US-2003-0044158-A1, the entire contentsof which are hereby incorporated by reference.

[0069] Photonic crystal fiber 120 has a circular cross-section, withcore 220 having a circular cross-section and region 210 (and layerstherein) having an annular cross-section. In other embodiments, however,the waveguide and its constituent regions may have different geometriccross-section such as a rectangular or a hexagonal cross-section.Furthermore, as mentioned below, core and confinement regions 220 and210 may include multiple dielectric materials having differentrefractive indices. In such cases, we may refer to an “averagerefractive index” of a given region, which refers to the sum of theweighted indices for the constituents of the region, where each index isweighted by the fractional area in the region of its constituent. Theboundary between region 220 and 210, however, is defined by a change inindex. The change may be caused by the interface of two differentdielectric materials or by different dopant concentrations in the samedielectric material (e.g., different dopant concentrations in silica).

[0070] Dielectric confinement region 210 guides EM radiation in a firstrange of wavelengths to propagate in dielectric core 220 along thewaveguide axis. The confinement mechanism is based on a photonic crystalstructure in region 210 that forms a bandgap including the first rangeof wavelengths. Because the confinement mechanism is not index-guiding,it is not necessary for the core to have a higher index than that of theportion of the confinement region immediately adjacent the core. To thecontrary, core 220 may have a lower average index than that ofconfinement region 210. For example, core 220 may be air, some othergas, such as nitrogen, or substantially evacuated. In such a case, EMradiation guided in the core will have much smaller losses and muchsmaller nonlinear interactions than EM radiation guided in a silicacore, reflecting the smaller absorption and nonlinear interactionconstants of many gases relative to silica or other such solid material.In additional embodiments, for example, core 220 may include a porousdielectric material to provide some structural support for thesurrounding confinement region while still defining a core that islargely air. Accordingly, core 220 need not have a uniform indexprofile.

[0071] The alternating layers 230 and 240 of confinement region 210 formwhat is known as a Bragg fiber. The alternating layers are analogous tothe alternating layers of a planar dielectric stack reflector (which isalso known as a Bragg mirror). The annular layers of confinement region210 and the alternating planar layers of a dielectric stack reflectorare both examples of a photonic crystal structure. Photonic crystalstructures are described generally in Photonic Crystals by John D.Joannopoulos et al. (Princeton University Press, Princeton N.J., 1995).

[0072] As used herein, a photonic crystal is a dielectric structure witha refractive index modulation that produces a photonic bandgap in thephotonic crystal. A photonic bandgap, as used herein, is a range ofwavelengths (or inversely, frequencies) in which there are no accessibleextended (i.e., propagating, non-localized) states in the dielectricstructure. Typically the structure is a periodic dielectric structure,but it may also include, e.g., more complex “quasi-crystals.” Thebandgap can be used to confine, guide, and/or localize light bycombining the photonic crystal with “defect” regions that deviate fromthe bandgap structure. Moreover, there are accessible extended statesfor wavelengths both below and above the gap, allowing light to beconfined even in lower-index regions (in contrast to index-guided TIRstructures, such as those described above). The term “accessible” statesmeans those states with which coupling is not already forbidden by somesymmetry or conservation law of the system. For example, intwo-dimensional systems, polarization is conserved, so only states of asimilar polarization need to be excluded from the bandgap. In awaveguide with uniform cross-section (such as a typical fiber), thewavevector β is conserved, so only states with a given β need to beexcluded from the bandgap to support photonic crystal guided modes.Moreover, in a waveguide with cylindrical symmetry, the “angularmomentum” index m is conserved, so only modes with the same m need to beexcluded from the bandgap. In short, for high-symmetry systems therequirements for photonic bandgaps are considerably relaxed compared to“complete” bandgaps in which all states, regardless of symmetry, areexcluded.

[0073] Accordingly, the dielectric stack reflector is highly reflectivein the photonic bandgap because EM radiation cannot propagate throughthe stack. Similarly, the annular layers in confinement region 210provide confinement because they are highly reflective for incident raysin the bandgap. Strictly speaking,.a photonic crystal is only completelyreflective in the bandgap when the index modulation in the photoniccrystal has an infinite extent. Otherwise, incident radiation can“tunnel” through the photonic crystal via an evanescent mode thatcouples propagating modes on either side of the photonic crystal. Inpractice, however, the rate of such tunneling decreases exponentiallywith photonic crystal thickness (e.g., the number of alternatinglayers). It also decreases with the magnitude of the index-contrast inthe confinement region.

[0074] Furthermore, a photonic bandgap may extend over only a relativelysmall region of propagation vectors. For example, a dielectric stack maybe highly reflective for a normally incident ray and yet only partiallyreflective for an obliquely incident ray. A “complete photonic bandgap”is a bandgap that extends over all possible wavevectors and allpolarizations. Generally, a complete photonic bandgap is only associatedwith a photonic crystal having index modulations along three dimensions.However, in the context of EM radiation incident on a photonic crystalfrom an adjacent dielectric material, we can also define an“omnidirectional photonic bandgap,” which is a photonic bandgap for allpossible wavevectors and polarizations for which the adjacent dielectricmaterial supports propagating EM modes. Equivalently, an omnidirectionalphotonic bandgap can be defined as a photonic band gap for all EM modesabove the light line, wherein the light line defines the lowestfrequency propagating mode supported by the material adjacent thephotonic crystal. For example, in air the light line is approximatelygiven by ω=cβ, where ω is the angular frequency of the radiation, β isthe wavevector, and c is the speed of light. A description of anomnidirectional planar reflector is disclosed in U.S. Pat. No.6,130,780, the contents of which are incorporated herein by reference.Furthermore, the use of alternating dielectric layers to provideomnidirectional reflection (in a planar limit) for a cylindricalwaveguide geometry is disclosed in U.S. Pat. No. 6,463,200, entitled“OMNIDIRECTIONAL MULTILAYER DEVICE FOR ENHANCED OPTICAL WAVEGUIDING,” toYoel Fink et al., the contents of which are incorporated herein byreference.

[0075] When alternating layers 230 and 240 in confinement region 210give rise to an omnidirectional bandgap with respect to core 220, theguided modes are strongly confined because, in principle, any EMradiation incident on the confinement region from the core is completelyreflected. However, such complete reflection only occurs when there arean infinite number of layers. For a finite number of layers (e.g., about20 layers), an omnidirectional photonic bandgap may correspond to areflection in a planar geometry of at least 95% for all angles ofincidence ranging from 0° to 80° and for all polarizations of EMradiation having frequency in the omnidirectional bandgap. Furthermore,even when photonic crystal fiber 120 has a confinement region with abandgap that is not omnidirectional, it may still support a stronglyguided mode, e.g., a mode with radiation losses of less than 0.1 dB/kmfor a range of frequencies in the bandgap. Generally, whether or not thebandgap is omnidirectional will depend on the size of the bandgapproduced by the alternating layer (which generally scales withindex-contrast of the two layers) and the lowest-index constituent ofthe photonic crystal.

[0076] In additional embodiments, the dielectric confinement region mayinclude photonic crystal structures different from a multilayer Braggconfiguration. For example, rather than the Bragg configuration, whichis an example of a one-dimensionally periodic photonic crystal (in theplanar limit), the confinement region may be selected to form, forexample, a two-dimensionally periodic photonic crystal (in the planarlimit), such as an index modulation corresponding to a honeycombstructure. See, for example, R. F. Cregan et al., Science 285, p.1537-1539, 1999. Furthermore, even in a Bragg-like configuration, thehigh-index layers may vary in index and thickness, and/or the low-indexlayers may vary in index and thickness. The confinement region may alsoinclude a periodic structure including more than two layers per period(e.g., three or more layers per period). Moreover, the refractive indexmodulation may vary continuously or discontinuously as a function offiber radius within the confinement region. In general, the confinementregion may be based on any index modulation that creates a photonicbandgap.

[0077] In the present embodiment, multilayer structure 210 forms a Braggreflector because it has a periodic index variation with respect to theradial axis. A suitable index variation is an approximate quarter-wavecondition. It is well-known that, for normal incidence, a maximum bandgap is obtained for a “quarter-wave” stack in which each layer has equaloptical thickness λ/4, or equivalently d_(H)/d_(L)=n_(L)n_(H), where dand n refer to the thickness and index, respectively, of the high-indexand low-index layers. These correspond to layers 240 and 230,respectively. Normal incidence corresponds to β=0. For a cylindricalwaveguide, the desired modes typically lie near the light line ω=cβ (inthe large core radius limit, the lowest-order modes are essentiallyplane waves propagating along z-axis, i.e., the waveguide axis). In thiscase, the quarter-wave condition becomes:$\frac{d_{H}}{d_{L}} = \frac{\sqrt{n_{L}^{2} - 1}}{\sqrt{n_{H}^{2} - 1}}$

[0078] Strictly speaking, this equation may not be exactly optimalbecause the quarter-wave condition is modified by the cylindricalgeometry, which may require the optical thickness of each layer to varysmoothly with its radial coordinate. Nonetheless, we find that thisequation provides an excellent guideline for optimizing many desirableproperties, especially for core radii larger than the mid-bandgapwavelength.

[0079] Some embodiments of photonic crystal fibers are described in U.S.patent application Ser. No. 10/057,258, entitled “LOW-LOSS PHOTONICCRYSTAL FIBER HAVING LARGE CORE RADIUS,” to Steven G. Johnson et al.,filed Jan. 25, 2002 and published under Pub. No. US-2002-0164137-A1, theentire contents of which are hereby incorporated by reference.

[0080] The radius of core 220 can vary depending on the end-useapplication of fiber 120. The core radius can depend on the wavelengthor wavelength range of the energy to be guided by the fiber, and onwhether the fiber is a single or multimode fiber. For example, where thefiber is a single mode fiber for guiding visible wavelengths (e.g.,between about 400 nm and 800 nm) the core radius can be in thesub-micron to several micron range (e.g., from about 0.5 μm to 5 μm).However, where the fiber is a multimode fiber for guiding IR wavelengths(e.g., from about 2 μm to 15 μm, such as 10.6 μm), the core radius canbe in the tens to thousands of microns range (e.g., from about 10 μm to2,000 μm, such as 500 μm to 1,000 μm). The core radius can be greaterthan about 5λ (e.g., more than about 10λ, 20λ, 50λ, 100λ), where λ isthe wavelength of the guided energy.

[0081] Two mechanisms by which energy can be lost from a guided signalin a photonic crystal fiber are by absorption loss and radiation loss.Absorption loss refers to loss due to material absorption. Radiationloss refers to energy that leaks from the fiber due to imperfectconfinement. Both modes of loss can be studied theoretically, forexample, using transfer matrix methods and perturbation theory. Adiscussion of transfer matrix methods can be found in an article by P.Yeh et al., J. Opt. Soc. Am., 68, p. 1196 (1978). A discussion ofperturbation theory can found in an article by M. Skorobogatiy et al.,Optics Express, 10, p. 1227 (2002). Particularly, transfer matrix codefinds propagation constants β for the “leaky” modes resonant in aphotonic crystal fiber structure. Imaginary parts of β's define themodal radiation loss, thus LOSS_(radiation)˜Im(β). Loss due to materialabsorption is calculated using perturbation theory expansions, and interms of the modal field overlap integral it can be determined from${{Loss}_{absorption} \sim {2{\pi\omega}{\int_{0}^{\infty}{r\quad {{r( {\alpha {\overset{harpoonup}{E}}_{\beta}^{*}{\overset{harpoonup}{E}}_{\beta}} )}}}}}},$

[0082] where ω is the radiation frequency, r is the fiber radius, α isbulk absorption of the material, and {right arrow over (E)}_(β) is anelectric field vector.

[0083] Based on theoretical and/or empirical investigations, photoniccrystal fibers, such as fiber 120, can be designed to minimize one orboth mode of loss. Guided modes can be classified as one of three types:pure transverse electric (TE); pure transverse magnetic (TM); and mixedmodes. Loss often depends on the type of mode. For example, TE modes canexhibit lower radiation and absorption losses than TM/mixed modes.Accordingly, the fiber can be optimized for guiding a mode thatexperiences low radiation and/or absorption loss. Alternatively, oradditionally, the fiber can be optimized for a mode that is well matchedto the mode of laser 110. For example, the fiber can be optimized forguiding the HE₁₁ (mixed) mode, which is well matched to the TEM₀₀ modeof a laser. Being “well matched” refers to efficient coupling betweenthe mode of the laser and the guided mode of the fiber.

[0084] Radiation loss can be reduced by adding layers to the confinementregion of fiber 120, increasing the index contrast between the high andlow index layers, increasing the core radius and/or lowering theintrinsic absorption losses of the first few layers by selectingmaterials with low absorption at the guided wavelengths. For example, atwavelengths of about 3 microns, chalcogenide glasses exhibit anabsorption coefficient of about 4 dB/m compared to many polymers whichhave an absorption coefficient of about 10⁵ dB/m in that wavelengthrange. Similarly, at 10.6 microns, chalcogenide glasses have anabsorption coefficient of about 10 dB/m compared to 10⁵ dB/m for manypolymers. Thus, using chalcogenide glasses instead of polymers canreduce losses in some cases. However, polymers, like oxide glasses, canprovide lower index materials than chalcogenide glasses.

[0085] As an example, consider a photonic crystal fiber having a coreradius R_(i)=500 μm, the confinement region materials have indices ofn_(l)=2.3 and n_(h)=2.7, with a bi-layer thickness, d=2.3 μm. Thecorresponding thickness of the low index and high index layers are 1.3μm and 1.0 μm, respectively. For the purposes of this example, theintrinsic bulk absorption loss of high/low index materials is taken tobe 10 dB/m. The support layer (R_(c)=1500 μm) is assumed to haveabsorption loss of 10⁵ dB/m, typical of polymers. The confinement regionhas 55 layers, thus R_(m)=563 μm.

[0086] At λ=10.6 μm, a theoretical model indicates that these structuralparameters define a fiber radiation loss of 24 dB/km (with a radiationloss decreasing by about an order of magnitude with every 30 layersadded to the confinement region), and a material absorption loss in theconfinement region of 0.23 dB/km. Adding 60 more layers to theconfinement region reduces radiation loss, which then becomes comparableto the material absorption loss in the mirror. These results aresummarized in FIG. 3A and FIG. 3B, which respectively show thedependence of the radiation and absorption losses on the operatingwavelength.

[0087] In contrast, consider a fiber having a similar structure, exceptwhere the low index and high index materials have refractive indices ofn_(l)=1.5 and n_(h)=2.8, with a bi-layer thickness of d=2.82 μm (thebi-layer refers to a high index and low index layer pair). Theserefractive index values are representative of a polymer low indexmaterial and a chalcogenide glass high index material. The correspondinglayer thicknesses are 1.97 μm and 0.84 μm for the low and high indexlayers, respectively. The intrinsic bulk absorption loss of high indexmaterial is 10 dB/m. The support layer (R_(c)=1500 μm) and low indexmaterial are assumed to have absorption loss of 10⁵ dB/m, typical ofpolymers. In this example, the confinement region is assumed to have 35layers (17.5 bi-layers), thus R_(m)=549 μm.

[0088] At λ=10.6 μm, these structural parameters define a fiberradiation loss of 1.09 dB/km (with a radiation loss decreasing by anorder of magnitude with every 4 bi-layers added), and a materialabsorption loss, in the mirror, of 320 dB/km, where power dissipationloss will be dominated by material absorption in the first few polymerlayers of the confinement region.

[0089] Accordingly, in some embodiments, the low index material can beselected to have low absorption loss in the first few layers of theconfinement region, and higher relative absorption loss in outer layers.The index contrast can be higher in the outer layers compared the innerlayers. For example, the confinement region can have low index layersthat include a chalcogenide glass in layers close to the core, butinclude a polymer or oxide glass in layers further from the core. Thehigh index layers can include a chalcogenide glass throughout.

[0090] As discussed previously, materials can be selected for theconfinement region to provide advantageous optical properties (e.g., lowabsorption with appropriate indices of refraction at the guidedwavelength(s)). However, the materials should also be compatible withthe processes used to manufacture the fiber. In some embodiments, thehigh and low index materials (e.g., the first and second chalcogenideglasses) should preferably be compatible for co-drawing. Criteria forco-drawing compatibility are provided in aforementioned U.S. patentapplication Ser. No. 10/121,452, entitled “HIGH INDEX-CONTRAST FIBERWAVEGUIDES AND APPLICATIONS.” In addition, the high and low indexmaterials should preferably be sufficiently stable with respect tocrystallization, phase separation, chemical attack and unwantedreactions for the conditions (e.g., environmental conditions such astemperature, humidity, and ambient gas environment) under which thefiber is formed, deployed, and used.

[0091] As mentioned in the foregoing description of fiber 120, layers240 and 230 can include a first and second chalcogenide glass,respectively (e.g., glasses containing a chalcogen element, such assulphur, selenium, and/or tellurium). In addition to a chalcogenelement, chalcogenide glasses may include one or more of the followingelements: boron, aluminum, silicon, phosphorus, sulfur, gallium,germanium, arsenic, indium, tin, antimony, thallium, lead, bismuth,cadmium, lanthanum and the halides (fluorine, chlorine, bromide,iodine).

[0092] Chalcogenide glasses can be binary or ternary glasses, e.g.,As—S, As—Se, Ge—S, Ge—Se, As—Te, Sb—Se, As—S—Se, S—Se—Te, As—Se—Te,As—S—Te, Ge—S—Te, Ge—Se—Te, Ge—S—Se, As—Ge—Se, As—Ge—Te, As—Se—Pb,As—S—Tl, As—Se—Tl, As—Te—Tl, As—Se—Ga, Ga—La—S, Ge—Sb—Se or complex,multi-component glasses based on these elements such as As—Ga—Ge—S,Pb—Ga—Ge—S, etc. The ratio of each element in a chalcogenide glass canbe varied.

[0093] The amount of the first chalcogenide glass in the high indexmaterial can vary. Typically, the high index material includes at leastabout 50% by weight of the first chalcogenide glass (e.g., at least 70%,80%, 90%, 95%, 98%, 99%). The high index material can be substantiallyexclusively chalcogenide glass (i.e., about 100% chalcogenide glass). Insome embodiments, in addition to the first chalcogenide glasses, thehigh index material can include one or more additional chalcogenideglasses, heavy metal oxide glasses, amorphous alloys, or combinationsthereof.

[0094] In some embodiments, the high index material is a chalcogenideglass including As and Se. For example, the high index material caninclude As₂Se₃. As₂Se₃ has a glass transition temperature (T_(g)) ofabout 180° C. and a thermal expansion coefficient (TEC) of about24×10⁻⁶/° C. At 10.6 μm, As₂Se₃ has a refractive index of 2.7775, asmeasured by Hartouni and coworkers and described in Proc. SPIE, 505, 11(1984), and an absorption coefficient, α, of 5.8 dB/m, as measured byVoigt and Linke and described in “Physics and Applications ofNon-Crystalline Semiconductors in Optoelectronics,” Ed. A. Andriesh andM. Bertolotti, NATO ASI Series, 3. High Technology, Vol. 36, p. 155(1996). Both of these references are hereby incorporated by reference intheir entirety.

[0095] The first chalcogenide glass can include As₂Se₃ and one or moreother elements. Examples of other elements that can be included are In,Sn, Sb, Te, I, Tl, Pb, and/or Bi. The index of the first chalcogenideglass can be greater than the refractive index of As₂Se₃. For example,chalcogenide glasses including Sb and/or Te in addition to As₂Se₃ canincrease the refractive index of the chalcogenide glass above therefractive index of As₂Se₃. The refractive index of the firstchalcogenide glass in these embodiments can be greater than about 2.8(e.g., more than 2.9, such as about 3.0 or more).

[0096] Some elements that can be added to As₂Se₃ to increase therefractive index of the first chalcogenide glass can change thethermomechanical properties of the first chalcogenide glass from thethermomechanical properties of As₂Se₃. The thermomechanical propertiesinclude phase transition temperatures, such as T_(g), and otherparameters such as the glass's TEC. For example, iodine may increase therefractive index of the first chalcogenide glass, but can reduce T_(g).In such cases, one or more additional compounds may be added to thefirst chalcogenide glass to mitigate the effects of the index-raisingelement on the glasses thermomechanical properties. On example of anelement that can reduce such thermomechanical effects is Ge.. Inembodiments, the second chalcogenide glass can have a T_(g) of more thanabout 180° C. (e.g., about 200° C., 220° C., 250° C. or more).

[0097] The amount of additional compounds added to As₂Se₃ in the firstchalcogenide glass can vary. Typically, the amount of various elementsin the first chalcogenide glass is determined empirically according tothe specifics of the photonic crystal fiber. For example, where thefiber design requires the first chalcogenide glass to have specificrefractive index, an amount of an index-raising element sufficient toprovide the desired index is added. Preferably, the amount of anyindex-raising element included will be sufficiently small to notsubstantially affect the stability of the glass (e.g., to prevent phaseseparation of the glass components). In some embodiments, the amount ofAs₂Se₃ in the first chalcogenide glass can be more than about 80% molar(e.g., more than about 90%, 95%, 99%) and the amount of one or moreadditional elements can be less than about 20% molar (e.g., less thanabout 10%, 5%, 1%).

[0098] The amount of the second chalcogenide glass in the low indexmaterial can vary. Typically, the low index material includes at leastabout 50% by weight of the second chalcogenide glass (e.g., at least70%, 80%, 90%, 95%, 98%, 99%). The low index material can besubstantially exclusively chalcogenide glass (i.e., about 100%chalcogenide glass). In some embodiments, in addition to the secondchalcogenide glasses, the high index material can include one or moreadditional chalcogenide glasses, heavy metal oxide glasses, amorphousalloys, or combinations thereof.

[0099] In some embodiments, the low index material is a chalcogenideglass including As and Se. For example, the high index material caninclude As₂Se₃.

[0100] The second chalcogenide glass can include As₂Se₃ and one or moreother elements. Examples of other elements that can be included are B,F, Al, Si, P, S, and/or Ge. In these embodiments, the index of thesecond chalcogenide glass can be less than the refractive index ofAs₂Se₃. For example, chalcogenide glasses including P and/or S inaddition to As₂Se₃ can reduce the refractive index of the chalcogenideglass below the refractive index of As₂Se₃. The refractive index of thesecond chalcogenide glass in these embodiments can be less than about2.7 (e.g., less than 2.5, such as about 2.0 or less).

[0101] Some elements that can be added to As₂Se₃ to reduce therefractive index of the second chalcogenide glass can change thethermomechanical properties of the first chalcogenide glass from thethermomechanical properties of As₂Se₃. For example, Si may reduce therefractive index of the second chalcogenide glass, and can increaseT_(g). In some such cases, one or more additional compounds may be addedto the second chalcogenide glass to mitigate the effects of theindex-reducing element to ensure the low index material is compatiblewith the high index material. In embodiments, the second chalcogenideglass can have a T_(g) of more than about 180° C. (e.g., about 200° C.,220° C., 250° C. or more).

[0102] The amount of additional compounds added to As₂Se₃ in the secondchalcogenide glass can vary. Typically, the amount of various elementsin the second chalcogenide glass is determined empirically according tothe specifics of the photonic crystal fiber. For example, where thefiber design requires the second chalcogenide glass to have specificrefractive index, an amount of an index-reducing element sufficient toprovide the desired index is added. Preferably, the amount of anyindex-reducing elements included will be sufficiently small to notsubstantially affect the stability of the glass (e.g., to prevent phaseseparation of the glass components). In some embodiments, the amount ofAs₂Se₃ in the second chalcogenide glass can be more than about 80% molar(e.g., more than about 90%, 95%, 99%) and the amount of one or moreadditional elements can be less than about 20% molar (e.g., less thanabout 10%, 5%, 1%).

[0103] In some embodiments, the second chalcogenide glass can includeAs₂S₃, GePS, and/or AsPS. The composition of the second chalcogenideglass including As₂S₃, GePS, and/or AsPS can be manipulated to obtain adesired refractive index and/or thermomechanical properties as describedfor As₂Se₃ above.

[0104] The first and/or second chalcogenide glasses can have relativelylow loss at a wavelength of interest compared to some non-chalcogenideglasses and/or some polymers (e.g., PES). For example, at 10.6 microns,the first and/or second chalcogenide glasses can have a lossco-efficient of less than about 1,000 dB/m. More preferably, the firstand/or second chalcogenide glasses can have a loss coefficient of lessthan about 50 dB/m, such as less than about 20 dB/m, 10 dB/m or less. Incontrast, polymers such as PES can have a loss co-efficient of 10,000dB/m or more.

[0105] In order for dielectric waveguides to function reliably at highpower densities, they should have low defect densities. In photoniccrystal fibers, such as those described herein, defects includedelamination between layers, cracking, or other structural defects, andmaterial defects, such as impurities. Selecting materials with matchedthermomechanical properties can reduce the occurrence of defects. Oneway to form preforms of these materials with high purity is to use CVD.

[0106] In embodiments where CVD is used, the high and low indexmaterials (e.g., the first and second chalcogenide glasses) should becompatible with this process. To be compatible with CVD, precursors forthe compounds from which solid deposits can be formed should beavailable for forming the high and low index materials.

[0107] Referring to FIG. 4, during the CVD process, a CVD system 500 isused to deposit layers of different materials on the inner surface of adeposition tube 501. CVD system 500 includes a gas source 510, a gasmanifold 520, and a lathe 530 on which deposition tube 501 is mounted.The material the system deposits in tube 501 forms in a chemicalreaction between gases supplied to tube 501 by gas source 510 viamanifold 520. System 500 also includes a microwave source 550, whichexcites a plasma in the gas within the tube, causing the gases to reactand deposit material on the tube surface. A furnace 540 heats tube 501to a desired temperature during the deposition process. System 500 alsoincludes tubes 570 that transport gases from gas source 510 to manifold520. Valves 580 modulate the flow of gases from gas source 510 tomanifold 520. The gases mix inside manifold 520 before being transportedto deposition tube 501 via a pipe 590. The deposition process iscontrolled by an electronic controller 560 (e.g., a system including aprocessor for executing instructions, such as a computer).

[0108] Referring also to FIG. 5, microwave source 550 includes aresonator enclosing a segment of deposition tube 501. During operation,the resonator couples microwave energy from a waveguide into gas (e.g.,vapor) within tube 501. Typically, this energy has a frequency in therange of about 1 to about 40 GHz. For example, the energy can have afrequency of about 5 to 15 GHz, such as about 12.5 GHz. The energygenerates a local non-isothermal low-pressure plasma region 610 withinthe tube. Gas flowing through the deposition tube is deflected by plasmaregion 610 to the space between plasma region 610 and tube 501, asindicated by arrows 620 and 630. Gasses proximate to the plasma reactwith each other, forming a layer of material one the inner surface oftube 501 adjacent plasma 610. Preferably, microwave energy istransferred without substantial energy loss to the tube itself, andmicrowave energy is coupled directly into the activated plasma insidethe tube.

[0109] During operation, system 500 translates microwave source 550 backand forth along the axis of tube 501, exciting plasma in the portion ofthe tube adjacent the source. Each pass of microwave source 500 relativeto the tube results in a layer of material being deposited within thetube. The microwave source 550 can be translated as many times asnecessary to provide the desired thickness of material with in the tube.

[0110] Furnace 540 heats the tube surface to a temperature sufficient toensure that deposited materials diffuse to form a consolidated layer.For this reason, the temperature depends upon the type of material beingdeposited. For many materials, the tube is heated to a temperaturebetween about 80° C. and 250° C., such as about 100° C. The tubetemperature is kept below a temperature that would cause any substantialadverse reaction in the deposited layer. For example, chalcogenideglasses may oxidize at temperatures above 250° C.-300° C. Thus, forthese glasses, the tube surface is maintained below these temperatures.Lower process temperatures can also reduce mechanical stress in thedeposited layers, reducing the possibility of fracture and/ordelamination in the multilayer structure. The tube surface temperaturemay be varied between depositing layers of different materials therein.

[0111] Controller 560 controls numerous parameters associated with thedeposition process to provide a layer of material having the desiredthickness and material properties (e.g., composition, density,homogeneity and/or layer morphology). These parameters include surfacetemperature, gas pressure, gas composition, microwave energy, andmicrowave frequency. The effects of the parameters on deposition rateand material properties are typically interrelated. For example, changesin gas pressure and/or gas composition can affect the deposition rate byproviding more or less of one or more reactant gases to the tube.Variations in microwave energy and/or frequency can vary the depositionrate by changing the temperature of the tube surface.

[0112] Due to its shape, plasma region 610 is often referred to as aplasma “ball.” The shape and size of the plasma ball is related to theplasma mode excited by the radiation and can be affected by gaspressure, the shape of the cavity, the gas composition, and/or theionization potential of the gas. For example, under otherwise equivalentconditions, the size of a plasma ball formed in nitrogen is typicallysmaller than a plasma ball formed in argon. Because the gas phasereaction of component gases occurs proximate to the plasma ball, theshape and size of the plasma ball can be selected to control the tubearea over which deposition occurs. In many embodiments, where thedeposition tube is cylindrical, the T₀₁ plasma mode is desirable.

[0113] Initially, a first gas composition is used to produce a layer ofa first material. After depositing the first material but prior todepositing the second material the tube is purged of residual reactivegases. Typically, the system flows an inert gas (i.e., inert withrespect to the layer of material just deposited in the tube and withresidual gases in the tube) through the tube for a time sufficient topurge substantially all of the first gas composition from the tube.Examples of inert gases include nitrogen and noble gases, such as argon.The system can monitor the composition of gas purged from the tube toestablish when the concentration of the first gas composition in thetube is sufficiently small to be negligible.

[0114] The first and second gas compositions include component gasesthat react upon heating by the plasma to form the first and secondmaterials, respectively. The type and relative concentration ofcomponent gases are selected based on the desired composition of thematerials. In embodiments where either of the materials are achalcogenide glass, at least one of the respective component gasesincludes a chalcogen element. In embodiments where either of thematerials is an oxide glass, the respective gas composition includesoxygen (e.g., as oxygen gas or the gas of an oxygen containingcompound). In each gas composition, one or more of the components can bea halide (e.g., a chloride) gas or a hydride gas. Examples of chloridesinclude SiCl₄, BCl₃, POCl₃, PCl₃, GeCl₄, SeCl₂, AsCl₃, and S₂Cl₂.Examples of hydrides include H₂Se, GeH₄, H₂S, H₂Te, AsH₃, and PH₃. Insome embodiments, chlorides may be preferred over hydrides, especiallywhere hydrogen and/or oxygen can contaminate the deposited material.Such contamination may occur where decomposition of the component gas isincomplete and/or due to the presence of water and/or oxygen.

[0115] During the deposition of a layer of the first or second material,the relative concentration of component gases can remain the same orvary. Where a homogeneous layer is desired, the relative concentrationof component gases is substantially constant. However, where variationsin composition are desired through the layer, the relative concentrationof component gases can vary during deposition of the layer. For example,where a refractive index gradient through the layer is desired, therelative concentration of component gases can be varied duringdeposition of the layer.

[0116] The first and/or second gas compositions can also include acarrier gas, which is inert with respect to the other component gases. Acarrier gas can be used to adjust the pressure of the first gascomposition without affecting the relative concentration of thecomponent gases. Carrier gases are selected based on the composition ofthe component gases. Examples of carrier gases include nitrogen andnoble gases, such as argon, and mixtures thereof.

[0117] The ratio of carrier gas to component (reactant) gas(es) in a gascomposition may vary as desired. Typically, the ratio of carrier tocomponent gas(es) is between about 1:10⁻⁴ and 1:10⁻¹. The relativeamount of component gas(es) to carrier gas can affect the depositionrate and the morphology of the deposited material.

[0118] In some embodiments, the first deposited layer may adverselyreact with a compound or element forming the subsequent layer while thatelement or compound is in the form of a gas. An adverse reactionintroduces impurities into the preform, which can be detrimental tofiber performance. For example, where an oxide glass is being depositedonto a layer of a chalcogenide glass, gaseous oxygen can oxidize thechalcogenide glass. In such instances, an inert component gas containingthe reactive element or compound can be chosen for the gas compositionto reduce (e.g., mitigate) any adverse reaction between the gas and thepreviously deposited layer (or tube). An example of a gas that can beused to provide oxygen when depositing an oxide glass on a chalcogenide(or other oxidizable glass) is nitrous oxide. In some embodiments, therelative concentration of the reactive gas (e.g., oxygen) can beincreased once a thin layer of material (e.g., oxide glass) has beendeposited on the previous layer.

[0119] Material may be deposited at relatively high rates. For example,the deposition rate may be about 1 μm/min or more (e.g., more than about5 μm/min, 8 μm/min, 10 μm/min).

[0120] In general, tube 501 can be formed from any material. Where thetube forms part of the final drawn fiber, the tube should be formed froma material that can be co-drawn with material deposited within the tube.In some embodiments, tube 501 is formed from a glass or a polymer.Examples of suitable glasses include silica-based glasses. Examples ofsuitable polymers include polysulfones, fluoropolymers (e.g., Teflon®),polyethylene and their derivatives.

[0121] Although microwave radiation is used to excite plasma in system100, other forms of EM radiation can also be used. For example, radiofrequency radiation (e.g., with frequencies less than about 109 Hz) canbe used to excite plasma in the tube. Furthermore, in some embodiments,plasma can be excited thermally alternatively or additionally to usingEM radiation.

[0122] To make a preform for a photonic crystal fiber, additional layersof material can be deposited on the layer of the second material. Insome embodiments, the sequential deposition of layers of the first andsecond materials is repeated multiple times (e.g., twice, three time,four times, or more). Alternatively, the composition of, e.g., a thirdlayer may differ from the composition of the first layer. For example,to make a preform for a low loss photonic crystal fiber, materials withhigh index contrast (e.g., layers of a chalcogenide glass and an oxideglass) can be deposited initially, followed by layers of materials withlow absorption (e.g., two different chalcogenide glasses). In someembodiments, many layers can be deposited (e.g., more than about 10layers, such as 20 or more layers).

[0123] The thickness of each layer may vary as desired. Generally, thedeposited layer thickness will depend on the desired structure of thewaveguide and draw ratio to be used in subsequent fiber drawing. Thethickness of alternating layers may be the same or different. In someembodiments, layers are formed that have the same optical thickness.Deposited layer thickness is typically between about 0.1 nm and 500 μm.

[0124] Although the CVD methods described herein are with reference tophotonic crystal fibers, they can also be used to make other types ofwaveguides (e.g., TIR optical fibers).

[0125] Referring to FIG. 6, in some embodiments, system 100 may bemodified to simultaneously provide output energy from laser 110 atmultiple locations. Modified system 700 includes a number of couplers710, which couple energy guided in waveguide 120 into other waveguides720. Each waveguide 720 can deliver laser energy to a different locationremote from laser 110. Waveguides 720 can be the same or different aswaveguide 120. For example, waveguides 720 can be photonic crystalfibers or some other type of waveguide (e.g., TIR fiber). The intensityof laser energy coupled into each waveguide 720 can be the same ordifferent. Where each waveguide's output is used in similarapplications, the intensity delivered by each waveguide can be the same.However, where applications are different, the delivered intensity canvary accordingly.

[0126] It will be understood that various modifications to the foregoingembodiments may be made without departing from the spirit and scope ofthe invention. Accordingly, other embodiments are within the scope ofthe following claims.

What is claimed is:
 1. A waveguide, comprising: a first portionextending along a waveguide axis comprising a first chalcogenide glass;and a second portion extending along the waveguide axis comprising asecond chalcogenide glass, wherein the second chalcogenide glass isdifferent from the first chalcogenide glass.
 2. The waveguide of claim1, wherein the first chalcogenide glass has a different refractive indexthan the second chalcogenide glass.
 3. The waveguide of claim 1, whereinthe first chalcogenide glass comprises As and Se.
 4. The waveguide ofclaim 3, wherein the first chalcogenide glass comprises As₂Se₃.
 5. Thewaveguide of claim 3, wherein the first chalcogenide glass furthercomprises Pb, Sb, Bi, I, or Te.
 6. The waveguide of claim 1 or 3,wherein the second chalcogenide glass comprises As and S.
 7. Thewaveguide of claim 6, wherein the second chalcogenide glass comprisesAs₂S₃.
 8. The waveguide of claim 1 or 3, wherein the second chalcogenideglass comprises P and S.
 9. The waveguide of claim 8, wherein the secondchalcogenide glass further comprises Ge or As.
 10. The waveguide ofclaim 1, further comprising a hollow core.
 11. The waveguide of claim 1,wherein the first chalcogenide glass has a refractive index of 2.7 ormore.
 12. The waveguide of claim 11, wherein the second chalcogenideglass has a refractive index of 2.7 or less.
 13. The waveguide of claim1, wherein the first chalcogenide glass has a T_(g) of about 180° C. ormore.
 14. The waveguide of claim 13, wherein the second chalcogenideglass has a T_(g) of about 180° C. or more.
 15. The waveguide of claim1, wherein the waveguide has a loss coefficient less than about 2 dB/mfor electromagnetic energy having a wavelength of about 10.6 microns.16. The waveguide of claim 1, wherein the first portion surrounds acore.
 17. The waveguide of claim 16, wherein the second portionsurrounds the core.
 18. The waveguide of claim 16, wherein the secondportion surrounds the first portion.
 19. The waveguide of claim 16,wherein the core has a minimum cross-sectional dimension of at leastabout 10 λ, where λ is the wavelength of radiation guided by thewaveguide.
 20. The waveguide of claim 19, wherein the minimumcross-sectional dimension of the core is at least about 20 λ.
 21. Thewaveguide of claim 16, wherein the core has a minimum cross-sectionaldimension of at least about 50 microns.
 22. The waveguide of claim 21,wherein the core has a minimum cross-sectional dimension of at leastabout 100 microns.
 23. The waveguide of claim 22, wherein the core has aminimum cross-sectional dimension of at least about 200 microns.
 24. Thewaveguide of claim 1, wherein the waveguide is a photonic crystal fiber.25. The waveguide of claim 24, wherein the photonic crystal fibercomprises a confinement region and the first and second portions arepart of the confinement region.
 26. The waveguide of claim 24, whereinthe photonic crystal fiber is a Bragg fiber.
 27. A method comprising:providing a waveguide comprising a first portion extending along awaveguide axis including a first chalcogenide glass and a second portionextending along the waveguide axis; and guiding electromagnetic energyfrom a first location to a second location through the waveguide. 28.The method of claim 27, wherein the second portion includes a secondchalcogenide glass different from the first chalcogenide glass.
 29. Themethod of claim 27, wherein the electromagnetic energy has a wavelengthof between about 2 microns and 15 microns.
 30. The method of claim 29,wherein the electromagnetic energy has a power of more than about oneWatt.
 31. The method of claim 30, wherein the electromagnetic energy hasa power of more than about 10 Watts.
 32. The method of claim 31, whereinthe electromagnetic energy has a power of more than about 100 Watts. 33.The method of claim 27, further comprising coupling the electromagneticenergy from a laser into the waveguide.
 34. The method of claim 33,wherein the laser is a CO₂ laser.
 35. The method of claim 27, whereinthe waveguide is a photonic crystal fiber.
 36. The method of claim 35,wherein the photonic crystal fiber is a Bragg fiber.
 37. An apparatus,comprising a dielectric waveguide extending along an axis and configuredto guide electromagnetic radiation along the axis, wherein theelectromagnetic radiation has a power greater than about 1 Watt.
 38. Theapparatus of claim 37, wherein the electromagnetic radiation has awavelength greater than about 2 microns.
 39. The apparatus of claim 38,wherein the electromagnetic radiation has a wavelength greater thanabout 5 microns.
 40. The apparatus of claim 37, wherein theelectromagnetic radiation has a wavelength less than about 20 microns.41. The apparatus of claim 40, wherein the electromagnetic radiation hasa wavelength less than about 15 microns.
 42. The apparatus of claim 39,wherein the electromagnetic radiation has a wavelength from about 10microns to 11 microns.
 43. The apparatus of claim 42, wherein theelectromagnetic radiation has a wavelength of about 10.6 microns. 44.The apparatus of claim 37, wherein electromagnetic radiation has a powergreater than about 5 Watts.
 45. The apparatus of claim 44, whereinelectromagnetic radiation has a power greater than about 10 Watts. 46.The apparatus of claim 45, wherein electromagnetic radiation has a powergreater than about 100 Watts.
 47. The apparatus of claim 37, wherein thedielectric waveguide comprises a first portion extending along thewaveguide axis comprising a first chalcogenide glass.
 48. The apparatusof claim 47, wherein the dielectric waveguide further comprises a secondportion extending along the waveguide axis, the second portion having adifferent composition than the first portion.
 49. The apparatus of claim48, wherein the second portion comprises a second glass different fromthe first chalcogenide glass.
 50. The apparatus of claim 49, wherein thesecond glass is a chalcogenide glass.
 51. The apparatus of claim 49,wherein the second glass is an oxide glass.
 52. The apparatus of claim37, wherein the waveguide is a photonic crystal fiber.
 53. The apparatusof claim 52, wherein the photonic crystal fiber is a Bragg fiber. 54.The apparatus of claim 37, wherein the waveguide comprises a hollowcore.